Several technologies that today predominantly support very high data rates are not well suited for IoT (Internet of Things), Energy Management, and Sensor applications. For this reason, several standardization development organizations like the Third Generation Partnership Project (3GPP) and the Institute of Electrical and Electronics Engineers (IEEE) are developing flavors of their mainstream technology that are optimized to support communication at longer range, but at lower data rates and, preferably, using less power consumption.
Recent interest has focused on so-called Long Range Low Power (LRLP) devices. As suggested by its name, LRLP devices are intended for operation at longer ranges and/or lower power than would be considered “normal” in the context of existing standards. LRLP is expected to be based on technologies and features found in IEEE 802.11ax, which is currently being standardized. That standardization is expected to accelerate the time to market for future LRLP products. One can imagine that access points supporting 802.11ax also have support for LRLP. Good coexistence between LRLP and 11ax devices will thus be important. A LRLP device, referred to as a “STA” or “station” in the standards jargon, is expected to operate on a much lower bandwidth (e.g. 2 MHz) as compared to the “legacy” 802.11 minimum bandwidth of 20 MHz. LRLP devices may be considered as belonging to a larger, or more general group of devices referred to as “narrow band Wi-Fi.”
In 802.11ax, a tone plan has been set for the new Fast Fourier Transform (FFT) size of 256, which is four times the size of the legacy FFT size. The smallest allocated sub band, called a resource unit or “RU.” consists of twenty-six subcarriers. Each RU contains two pilot tones. The largest tone unit for 20 MHz contains 234 tones and 8 pilot tones. There are many more tone unit sizes for different bandwidths. This tone plan is required for resource allocation with OFDMA in uplink and downlink.
It is recognized herein that certain challenges arise in an environment where both 802.11ax STAs and LRLP STAs are to be served. More generally, it is recognized herein that the same or similar challenges arise whenever a wireless access point must support devices intended to operate at longer distances and/or lower powers than are considered “normal” for the involved air interface standard.
As an example, LRLP STAs may have substantially different cyclic-prefix length requirements than 802.11ax STAs on the downlink. For example, a LRLP STA far from the involved wireless access point may require a much longer cyclic prefix than is required by an 802.11ax STA operating relatively close to the wireless access point. One solution is to use a long cyclic prefix for all the sub bands in the OFDMA transmission. However, this approach increases the overhead and therefore wastes capacity and it is not possible to use different lengths of cyclic prefixes in the same OFDMA symbol without causing inter symbol interference (ISI).
Another problem with co-scheduling LRLP and 802.111ax STAs in an OFDMA fashion in the downlink is the need of a narrowband preamble. There is no simple way for an LRLP STA to use parts of the legacy preamble of 20 MHz to perform time and frequency synchronization.
Because LRLP STAs only support lower bandwidths and need to filter out their allocated sub bands, e.g., an allocated 2 MHz sub band within a 20 MHz OFDMA signal. That operation is a difficult problem to achieve if an LRLP STA samples at a much lower rate than 20 MHz. Any receive filter applied to an OFDM signal will effectively shorten the cyclic prefix due to the variations in the group delays of the applied filters, which may introduce ISI in the signal.